We demonstrate that many-body localization of two-dimensional weakly interacting bosons in disorder remains stable in the thermodynamic limit at sufficiently low temperatures. Highly energetic particles destroy the localized state only above a critical temperature, which increases with the strength of the disorder. If the particle distribution is truncated at high energies, as it is for cold-atom systems, the localization can be stable at any temperature.

We investigate the ultrafast nonlinear dynamics of light propagating in an active plasmonic waveguide composed of a thin film of gold sandwiched by two silicon layers immersed in externally pumped ${\mathrm{Al}}_2{\mathrm{O}}_3$:${\mathrm{Er}}^{3+}$. We model optical propagation in such system through a generalized Ginzburg-Landau equation, finding that heating affects the temporal dynamics of dissipative solitons by inducing a self-frequency redshift accompanied by deceleration in the time domain. We further evaluate the dependence of the self-induced redshift by developing a semianalytical variational approach, providing analytical predictions that are in excellent agreement with direct numerical simulations of the generalized Ginzburg-Landau equation. Our results provide a general understanding of ultrafast nonlinear dynamics in gold-based active plasmonic waveguides, as in particular the spectral shaping properties of propagating optical pulses.

A universal relation has been established between the local energy transport velocity along the direction of propagation and the group velocity of scalar and vector-valued propagation-invariant spatiotemporally localized superluminal and subluminal electromagnetic waves in free space. Under specific restrictions, this relationship is very closely valid for physically realizable almost propagation-invariant spatiotemporally confined subluminal and superluminal electromagnetic fields. In both cases, although the group velocity may be either superluminal or subluminal, the universal relation is in accord with the well-established result that the upper limit of the energy transport velocity is $c$, the speed of light in vacuum.

Ab initio calculations of QED radiative corrections to the ${}^{2}P_{1/2}\text{−}{}^{2}P_{3/2}$ fine-structure transition energy are performed for selected F-like ions. These calculations are nonperturbative in $\alpha Z$ and include all first-order and many-electron second-order effects in $\alpha$. When compared to approximate QED computations, a notable discrepancy is found especially for F-like uranium for which the predicted self-energy contributions even differ in sign. Moreover, all deviations between theory and experiment for the ${}^{2}P_{1/2}\text{−}{}^{2}P_{3/2}$ fine-structure energies of F-like ions, reported recently by [Li et al., Phys. Rev. A 98, 020502(R) (2018)], are resolved if their highly accurate, non-QED fine-structure values are combined with the QED corrections ab initially evaluated here.

We extend the Wigner current vector field (Wigner current) construct to single bosonic mode quantum systems interacting with an environment. In terms of the Wigner function quasiprobability density and associated Wigner current, the open system quantum dynamics can be concisely expressed as a continuity equation. Through the consideration of the harmonic oscillator and additively driven Duffing oscillator in the bistable regime as illustrative system examples, we show how the evolving Wigner current vector field on the system phase space yields useful geometric insights concerning how quantum states decohere away due to interactions with the environment, as well as how they may be stabilized through the counteracting effects of the system anharmonicity (i.e., nonlinearity).

We discuss interferometric parameter estimation of the amplitude, instead of the phase, via weak value amplification. The considered weak interaction introduces modulation on the amplitude of the wave function; therefore, the two-party state experiences a nonunitary evolution. With the same pre- and postselection states as those of original weak value amplification, a much larger anomalous amplification factor can be attained. The shift in the intensity profile at the dark port and the signal-to-noise ratio for the parameter of interest get further amplified by the weak value, compared to phase-based weak value amplification. Although the nonunitary evolution introduces loss, more information about the parameter of interest can be extracted. Thus the weak value amplification is extended to measurement of amplitude and nonunitary processes. We also discuss possible applications of this idea for precisely measuring tiny rotations or enhancing the image contrast of transparent objects.

We introduce a measure of quantum non-Gaussianity (QNG) for those quantum states not accessible by a mixture of Gaussian states in terms of quantum relative entropy. Specifically, we employ a convex-roof extension using all possible mixed-state decompositions beyond the usual pure-state decompositions. We prove that this approach brings a QNG measure fulfilling the properties desired as a proper monotone under Gaussian channels and conditional Gaussian operations. As an illustration, we explicitly calculate QNG for the noisy single-photon states and demonstrate that QNG coincides with non-Gaussianity of the state itself when the single-photon fraction is sufficiently large.

An artificial neural network, consisting of many neurons in different layers, is an important method to simulate the human brain. Usually, one neuron has two operations: one is linear, the other is nonlinear. The linear operation is the inner product and the nonlinear operation is represented by an activation function. In this work, we introduce a kind of quantum neuron whose inputs and outputs are quantum states. The inner product and activation operator of the quantum neurons can be realized by quantum circuits. Based on the quantum neuron, we propose a model of a quantum neural network in which the weights between neurons are all quantum states. We also construct a quantum circuit to realize this quantum neural network model. A learning algorithm is proposed meanwhile. We show the validity of the learning algorithm theoretically and demonstrate the potential of the quantum neural network numerically.

We show that the set of not-completely-positive (NCP) maps is unbounded, unless further assumptions are made. This is done by first proposing a reasonable definition of a valid NCP map, which is nontrivial because NCP maps may lack a full positivity domain. The definition is motivated by specific examples. We prove that for valid NCP maps, the eigenvalue spectrum of the corresponding dynamical matrix is not bounded. Based on this, we argue that in general the volume measure of qubit maps, including NCP maps, is not well defined.

More than 40 years ago, Barash published a calculation of the full retarded Casimir-Lifshitz torque for planar birefringent media with arbitrary degrees of anisotropy. An independent theoretical confirmation has been lacking since. We report a systematic and transparent derivation of the torque between two media with both electric and magnetic birefringence. Our approach, based on an eigenmode decomposition of Maxwell's equations, generalizes Barash's result for electrically birefringent materials, and can be generalized to a wide range of anisotropic materials and finite thickness effects.

We analyze the ground-state properties of three-body constrained bosons in a one-dimensional optical lattice with staggered hoppings analogous to the double-well optical lattice. By considering attractive and repulsive on-site interactions between the bosons, we obtain the phase diagram which exhibits various quantum phases. Due to the double-well geometry and three-body constraint, several gapped phases such as the Mott insulators and dimer–bond-order phases emerge at commensurate densities in the repulsive interaction regime. Attractive interaction leads to the pair formation which leads to the pair–bond-order phase at unit filling, which resembles the valence-bond solid phase of composite bosonic pairs. At incommensurate densities, we see the signatures of the gapless pair superfluid phase.

We study the dynamics of multiple $\mathrm{\Xi }$-type atoms driven by a squeezed vacuum reservoir in a quasi-one-dimensional waveguide. We show that the atomic system's steady state is a pure state, and a complete population inversion can occur when the dipole moment of the second transition is almost perpendicular to the polarization of the incident squeezed light. We also prove that the steady state of the system is the direct product of that in the single-atom case with modified squeezed vacuum even when dipole-dipole interaction is involved. This steady-state population inversion may be used to study the two-photon laser or collective atomic effect.

We propose a double-$\mathsf{M}$ system of cold atoms for realizing the reversible storage and manipulation of signal fields encoded in degrees of freedom (DOFs) of polarization and orbital angular momentum (OAM). By employing two pairs of counterpropagating controlling fields, we show that the system exhibits dark state polaritons relating to the polarization and propagation direction of the light fields. Through the active operation of the controlling fields in time, stationary light pulses with polarizations and OAM information can be generated due to the tight coupling and balanced competition between the retrieved signal fields, and can be manipulated coherently and all-optically. Another advantage of our proposed scheme is that the two polarization components of the signal field can be decoupled and thus be independently retrieved from the atomic ensemble, and then the scheme can be employed to realize polarization-dependent beam splitting. Such a multiple DOF-dependent scheme could pave the way toward quantum nonlinear optics without a cavity, and could greatly enhance the tunability and capacity for both classical and quantum information processing.

We develop a fully spatiotemporal numerical model, based on stochastic simulations, simulating the generation of spatiotemporal multimode spontaneous parametric down-conversion, the propagation of the signal and idler beams through a Hong-Ou-Mandel (HOM) interferometer, and the detection of the outgoing beams with two separate detectors arrays. Spatial and temporal properties of the two-photon interference are investigated by measuring the spatial distribution of the momentum correlations between the time-integrated outgoing far-field images and a full spatiotemporal HOM dip is exhibited. Numerical results also demonstrate that the spatiotemporal coherence properties of biphoton wave packets can be fully characterized by detectors arrays with no temporal resolution.

Different from classical Talbot self-imaging, second-order spatial Talbot effect is a kind of second-order imaging, in which the self-imaging of an object is observed through intensity correlation measurement. Here, in terms of space-time duality, we investigate the temporal counterpart of second-order Talbot self-imaging with temporally incoherent light. Our work shows that second-order Talbot effect in the time domain is related to the joint dispersion of light pulse in dispersive medium. Particularly, based on the discrete Fourier transformation, the fractional Talbot self-imaging via intensity correlation doesn't generally possess a definite fractional period, which contradicts some articles' results. Our results may be useful for implementing the repetition-rate-multiplication technique and remotely transmitting the periodic temporal signals robustly.

The immense scalability of continuous-variable cluster states motivates their study as a platform for quantum computing, with fault tolerance possible given sufficient squeezing and appropriately encoded qubits [N. C. Menicucci, Phys. Rev. Lett. 112, 120504 (2014)]. Here, we expand the scope of that result by showing that additional antisqueezing has no effect on the fault-tolerance threshold, removing the purity requirement for experimental continuous-variable cluster-state quantum computing. We emphasize that the appropriate experimental target for fault-tolerant applications is to directly measure 15–17 dB of squeezing in the cluster state rather than the more conservative upper bound of 20.5 dB.

We present photoexcitation of ultra-long-range Rydberg molecules as a probe of spatial correlations in bosonic and fermionic quantum gases. Rydberg molecules can be created with well-defined internuclear spacing, set by the radius of the outer lobe of the Rydberg electron wave function $R_n$. By varying the principal quantum number $n$ of the target Rydberg state, the molecular excitation rate can be used to map the pair-correlation function of the trapped gas $g^{\left(2\right)}\left(R_n\right)$. We demonstrate this with ultracold Sr gases and probe pair-separation length scales in the range $R_n=1400–3200$ $a_0$, which are on the order of the thermal de Broglie wavelength for temperatures around 1 $\mu \mathrm{K}$. We observe bunching for a single-component Bose gas of ${}^{84}\mathrm{Sr}$ and antibunching due to Pauli exclusion at short distances for a polarized Fermi gas of ${}^{87}\mathrm{Sr}$, revealing the effects of quantum statistics.

In quantum mechanics, joint measurements of noncommuting observables are only possible if a minimal unavoidable measurement uncertainty is accepted. On the other hand, correlations between noncommuting observables can exceed classical limits, as demonstrated by the violation of Bell's inequalities. Here, the relation between the uncertainty-limited statistics of joint measurements and the limits on expectation values of possible input states is analyzed. It is shown that the experimentally observable statistics of joint measurements explain the uncertainty limits of local states, but result in less restrictive bounds when applied to identify the limits of nonlocal correlations between two separate quantum systems. A tight upper bound is obtained for the four correlations that appear in the violation of Bell's inequalities and the statistics of pure states saturating the bound is characterized. The results indicate that the limitations of quantum nonlocality are a necessary consequence of the local features of joint measurements, suggesting the possibility that quantum nonlocality could be explained in terms of the local characteristics of quantum statistics.